The present invention relates to implantable cardiac stimulating devices, including devices having defibrillation or cardioversion leads and, more particularly, concerns an implantable cardiac stimulating device that is adapted to obtain an accurate impedance measurement of the cardioversion or defibrillation lead using a low amplitude, high frequency signal.
Implantable cardiac stimulating devices are devices that are adapted to be implanted within the body of a patient so that therapeutic electrical stimulation can be provided to the patient""s heart to regulate heart function. These types of devices include well known pacemakers or implantable cardioverter defibrillators (ICDs) or devices that include the functionality of both a pacemaker and an ICD.
Typically, these devices include a control unit, having a microprocessor, and one or more leads that are adapted to be positioned adjacent the walls of the heart. The control unit typically receives sensory input about the function of the heart and, when the input is indicative of a heart arrhythmia, the control unit then provides an appropriate therapeutic electrical stimulation to the heart via the leads. The therapeutic electrical stimulation can, for example, consist of a low voltage pacing pulse to ensure the heart is beating correctly, or can include a high-voltage waveform that is adapted to terminate a particular form of cardiac arrhythmia, such as ventricular fibrillation.
One particular problem with implantable cardiac stimulating devices is that the lead that is adapted to provide the electrical stimulation to the heart can become damaged. In many instances, the leads are implanted into the chambers of the heart. In this environment, the leads are continuously subjected to forces as a result of the beating of the heart. Over time, the leads can become damaged or even broken such that the delivery of the therapeutic electrical stimulation can be impaired. This is a very serious problem with ICDs that are adapted to terminate more serious forms of cardiac arrhythmia.
For example, if the ICD is adapted to recognize and provide a therapeutic shock to the heart upon the occurrence of ventricular fibrillation, a damaged or broken lead may result in the ICD being unable to provide a waveform of sufficient magnitude to terminate the ventricular fibrillation. As is generally understood, ventricular fibrillation is characterized by the random depolarization of the cardiac cells in the ventricle. The random depolarization of heart cells results in little or no blood being pumped by the heart which can ultimately cause the death of the patient. Typically, upon recognizing ventricular fibrillation, the ICD provides a high voltage, e.g., 550 volt, biphasic waveform to the heart that simultaneously depolarizes the majority of the heart cells so that the cells can simultaneously repolarize and, desirably, begin depolarizing in a more synchronous fashion.
An increase in the impedance of the lead that is to deliver the defibrillation waveform may result in a degradation in the amplitude of the waveform such that the waveform may be unable to terminate the arrhythmia. In extreme cases, a broken lead may result in the ICD being unable to deliver any high voltage waveform to the heart. Consequently, it is desirable to be able to periodically assess the impedance of a high voltage defibrillation or cardioversion lead to ensure that the lead will be able to adequately provide the high voltage waveform to terminate a life threatening arrhythmia.
To address these particular problems, some implantable cardiac devices of the prior art have instituted procedures whereby the impedance of leads are periodically measured. Once such example is provided by U.S. Pat. No. 5,549,646 to Katz et al. The device disclosed in this patent includes an impedance measurement circuit that has a voltage source which applies a voltage to an ICD lead so that impedance measurement of the ICD lead can be obtained. The impedance measurement of the ICD lead can then be compared to a reference value to determine whether the lead impedance has exceeded a predetermined amount. However, the circuit disclosed in this patent uses a low voltage, low frequency source in order to determine the lead impedance. While the use of the low voltage source reduces the amount of limited power that is consumed in order to test the impedance, this low voltage source will give rise to an impedance measurement that does not necessarily correspond to the impedance that would occur when the high voltage defibrillation or cardioversion waveform is applied across the leads.
Specifically, the lead impedance at voltages below 100 volts is a non-linear function of the electrode surface area and the amplitude of the current passing through the electrode. The non-linearity of the relationship between the impedance, the surface area and the current is due to the capacitive polarizing effects and the electrochemical reactions at the lead-tissue interface. The impedance that is obtained using the lead impedance measurement circuit of U.S. Pat. No. 5,549,646 does not always yield a measurement value that can be used to determine what the corresponding impedance would be when a high voltage cardioversion or defibrillation waveform is applied to the heart. As a consequence, using low voltage, low frequency waveforms generally does not result in an impedance measurement that accurately reflects the actual lead impedance. Hence, the impedance reference values have a degree of error which may result in the impedance measurement circuit obscuring subtle changes in impedance which precede total lead failure, disabling effective high voltage therapy to the heart to correct an arrhythmia.
As an alternative, the impedance can also be measured using a high voltage waveform similar to the cardioversion or defibrillation waveform. However, for testing purposes this is impractical as it would cause tremendous discomfort to the patient, create a risk of inducing unwanted cardiac arrhythmias and consume a significant amount of limited battery power.
Therefore, there is a need for an implantable cardiac stimulating device that can periodically assess the status of the leads and the ability of the leads to deliver a high voltage therapeutic waveform. To this end, there is a need for an implantable cardiac stimulating device that is capable of obtaining an impedance measurement using a low voltage waveform that is indicative of the corresponding impedance that will occur when a high voltage therapeutic waveform is applied to the heart from the leads to correct an arrhythmia.
The aforementioned needs are satisfied by the implantable cardiac stimulating device of the present invention which is comprised of a control unit that is adapted to be implanted within the heart of a patient and at least one high voltage lead that is adapted to be positioned adjacent the heart so as to apply high voltage cardioversion or defibrillation shocks to the heart. The control unit also includes an impedance measurement circuit which is adapted to be able to provide a low amplitude, high frequency impedance measurement signal to the lead and then measure the resulting electrical response on a second electrode so that an impedance measurement can be obtained using a low amplitude signal. The resulting impedance measurement has a high correlation to the actual impedance that would occur when a high voltage cardioversion or defibrillation waveform is applied to the lead. In this way, the impedance of the particular lead can be accurately measured without consuming excess power and without being felt by the patient, while still obtaining a measurement that has a high correlation to the impedance that would actually occur when the high voltage cardioversion or defibrillation waveform is applied to the lead.
In one embodiment, a 50 KHz sinusoidal constant current signal having a magnitude of approximately either 100 or 500 microamps, peak to peak, is enabled by a microprocessor in the control unit so as to provide the impedance measurement signal between two electrodes for less than one second to a defibrillation lead. The resulting voltage between these electrodes, which can comprise any of a number of electrodes including the casing of the implantable cardiac device, is then measured in order to obtain a measurement of the impedance of the lead. Applying such a high frequency, low amplitude current for a period of less than one second results in the ability to test the impedance and obtain an impedance measurement that is within a few percent of the impedance that would be measured when a high energy (e.g., 16 joule) defibrillation waveform is applied to the heart. The use of such a high frequency signal avoids the polarization error that would otherwise occur in the measurement of the impedance.
It will be appreciated that the implantable cardiac device incorporating the impedance measurement circuit of the present invention is capable of measuring the impedance and providing a very accurate indication of the impedance that will occur upon the delivery of a high voltage cardioversion or defibrillation waveform without requiring the consumption of a large amount of limited battery power and without being felt by the patient. These and other objects and advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.